1. Field of the Invention
The present invention relates generally to semiconductor lasers, and more particularly to temperature increase of tunable Vertical Cavity Surface Emitting Lasers (VCSELs). The invention describes an approach developed to reduce the device temperature and consequently improve its performance.
2. Description of Related Art
Optical communication systems are a substantial and fast growing constituent of communications networks. Such optical systems include, but are not limited to, telecommunication systems, cable television systems, and Local Area Networks (LANs). Optical systems are described in Gowar, Ed. Optical Communication Systems, (Prentice Hall, N.Y.) c. 1993, the disclosure of which is incorporated herein by reference. Currently, the majority of optical systems are configured to carry an optical channel of a single wavelength over one or more optical wave-guides. To convey the information form plural sources, time division multiplexing is frequently employed (TDM). In time division multiplexing, a particular time slot is assigned to each information source, the complete signal being constructed from the signal collected from each time slot. While this is a useful technique for carrying plural information sources on a s single channel, its capacity is limited by fiber dispersion and the need to generate high peak power pulses.
While the need for communication systems increases, the current capacity of existing wave-guiding media is limited. Although capacity may be expanded, e.g. by laying more fiber optic cables, the cost of such expansion is prohibitive. Consequently, there exists a need for a cost-effective way to increase the capacity of the existing optical wave-guides.
Wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) have been explored as approaches for increasing the capacity of the existing fiber optic networks. Such system employs plural optical signal channels, each channel being assigned a particular channel wavelength. In a typical system, optical signal channels are generated, multiplexed to form an optical signal comprised of the individual optical signal channels, transmitted over a single wave-guide, and de-multiplexed such that each channel wavelength is individually routed to a designated receiver. Through the use of optical amplifiers, such as doped fiber amplifiers, plural optical channels are directly amplified simultaneously, facilitating the use of WDM and DWDM approaches in long distance optical systems.
Crucial to providing sufficient bandwidth for WDM and DWDM, while at the same time avoiding bottlenecks, is the ability to assign and re-assign wavelengths as needed throughout the network and providing the bandwidth when and where needed. Allowing more flexibility in the way fiber capacity is provisioned is the driving force behind the requirements of next generation optical networks. Future network capacity needs will probably require a multi fold scalability beyond a network's initial installed capacity and also a rapid service activation to allow high capacity links to be deployed as needed.
Tunable lasers that can be tuned over a wide range of wavelengths and switched at nanosecond speeds best meet such requirements. A number of schemes have been proposed and studied to obtain frequency tuning of semiconductor lasers. These methods have typically relied on tuning the index of refraction of the optical cavity. The resulting tunable range is, however, restricted to approximately 10 nm.
In addition, the bulk of the tuning schemes have been attempted with edge emitting laser structures. Unlike vertical cavity surface emitting lasers (VCSEL), these structures are not single mode and consequently the use of distributed Bragg reflectors or distributed feedback, both of which are difficult to fabricate, are required to select a single mode.
Interferometric techniques that rely on variable selection of different Fabry-Perot modes for tuning from a comb of modes have also been proposed. Among these are asymmetric y-branch couplers and vertical cavity filters. These methods produce tuning ranges of up to 100 nm, but are, however, restricted to discrete tuning only and are potentially unstable between the tuning steps.
Most of the above mentioned techniques are polarization sensitive and therefore cannot be readily adopted to optical communications systems, which need to be robust and inexpensive and consequently insensitive to beam polarization.
A critical and costly problem in all WDM and DWDM is created by the need for exact wavelength registration between transmitters and receivers. A tunable receiver capable of locking to the incoming signal over a range of wavelengths variation would relax the extremely stringent wavelength registration problem. The tunability requirement can best be met by proper VCSEL utilization. VCSELs possess desirable qualities for telecommunications: circular mode profile that makes them ideally suited for coupling into optical fibers, single mode operation, surface mode operation and compact size. Complete description of the VCSEL device and its operation can be found in the U.S. Pat. Nos. 5,629,951 and 5,771,253 both of which are incorporated herein by reference.
VCSEL of this invention is a cantilever apparatus based on the principle of an electrostatic force pulling on a simple cantilever arm. The device so formed is capable of continuously tuning the resonant frequency of the Fabry-Perot cavity over a wide range of wavelengths. The resonant cavity is formed between two distributed Bragg reflector (DBR) mirrors. The top reflector is composed of a movable top DBR supported in a cantilever arm, a variable thickness air spacer layer and a fixed DBR. The bottom reflector is fixed in the substrate. By applying a tuning voltage to create electrostatic attraction, the cantilever arm may be deformed towards the substrate, thereby changing the thickness of the air spacer layer and consequently the resonant wavelength of the Fabry-Perot cavity. A precise control of substrate to cantilever arm distance is necessary in order to maintain the desired wavelength and meet the wavelength stability requirements.
One of the important advantages of semiconductor lasers is that they can be directly modulated, i.e. one can readily obtain short optical pulses useful for optical communications by modulating the laser current. This is typically accomplished by placing a small amplitude modulated alternating current signal onto a direct current signal. Typical laser frequency response (transfer function) showing the output power as a function of frequency is shown in FIG. 1. The power peak is observed at the laser's resonant frequency. The output power is strongly affected by the laser parasitic capacitance's. The adverse effect of the increasing parasitic capacitance on the output power is shown in FIG. 2. At higher operating frequencies this effect becomes even more pronounced. In a typical VCSEL, the primary contributors to the parasitic capacitance are the tuning pad, laser head and the bonding pad. Semiconductor laser manufacturers have resorted to creative but costly approaches to reduce the effects of parasitic capacitance. Most typically, a deep proton implants are utilized to electrically isolate the regions contributing to the capacitance. The deep implant is intended to isolate the laser head from the tuning pad. The shallow implant defines the aperture around the laser head. Since the layers that require implant are several microns thick and the background concentration that needs to be overcome for the isolation to be effective is of the order of 10exp18, a high energy and a high dose proton implants are mandatory. Typically, a deep implant requires energies of the order of 1 mega electron volts (MeV). In spite of high proton beam energies, due to normal process variations, the process may not succeed in completely isolating the implanted areas, i.e., some portion of the background doping of semiconducting layers may not be neutralized and a current path may exist between the areas intended to be isolated from each other. Moreover, inherent to the implant process is an additional photolithography process step requiring a photoresist layer to protect the areas of the device not to be implanted. The photoresist thickness needs to be approximately 25 micrometers (um). As a result of high proton beam energy and the time required to complete the deep implant process, the photoresist layer is exposed to very high temperature for long periods of time that cause it to polymerize and change to a very hardened state. Numerous dry and wet photoresist stripping steps need to be implemented to successfully remove the photoresist after an implant step.
Additionally, the deep implant has several drawbacks. It causes substantial damage to the implanted areas of the device that cannot be annealed out in this application since the anneal process would electrically activate the implanted region and defeat the main purpose of the implant process. The high energy of a deep implant also damages the cantilever and makes it more droop prone. The cantilever droop would causes a shift in the device resonant wavelength and a possible failure of communications equipment the laser is installed in. Furthermore, the process cycle time due to the implant and the subsequent photoresist stripping step is substantially increased as several process steps are added and the cost of an implant run is of the order of several thousand dollars. The device reliability due to the damage to cantilever and other areas of the device may also be compromised.
Due to the deficiencies of the existing technologies described here there is a need for an improved process that meets electrical isolation requirements and avoids costly photolithography and implant process steps. The invention disclosed herein meets such requirements.